Amid the boxes of ammunition and gun manuals investigators found in Newtown shooter Adam Lanza's home were journals and drawings that experts say could provide the most useful clues to why he killed 20 children and six adults at Sandy Hook Elementary School.

"The journals may explain a lot about Adam Lanza, such as what made him mad or what made him happy, and when things didn't go his way in school or at home, how did he deal with that?'' said forensic psychologist Stanton E. Samenow, the author of "Inside the Criminal Mind," who recently interviewed and analyzed Washington, D.C. sniper, Lee Boyd Malvo.

State police found seven journals they say were written by Lanza, as well as some of his drawings, according to search warrants unsealed Thursday. The warrants do not indicate when the journals were written or when the drawing were made. They also do not provide details of what was written or drawn.

Investigative sources have said Lanza did not leave behind a note or manifesto. On Dec. 14, he shot and killed his mother, Nancy Lanza, in their Newtown home before shooting 20 first-graders and six adults at the school. He killed himself as police moved in.

"Even if the journals don't have any details about how he planned this massacre, they can explain a lot about how he saw himself and, just as importantly, how he saw others,'' Samenow said.

Four boxes of Lanza's writings, drawings, personal memorabilia, books, family photos, school papers and a military-style uniform were shipped to the FBI's Behavioral Analysis Unit, which has been working closely with state police.

It is unclear if FBI profilers will prepare an analysis of Lanza for the state police to include in their final report on the Sandy Hook massacre, which is expected to be released in a couple of months. The agency also could use the items to develop future methods of identifying mass murderers before they strike, experts said.

A former FBI profiler said the agency will look for clues into what turned Lanza callous, calculated killer.

"He just didn't decide a day before to do this," said the profiler, Mary Ellen O'Toole. "There had to be anger and hatred building for awhile. He exhibited an amazing amount of callousness, firing away while children were screaming. You don't get that overnight."

O'Toole spent years analyzing school shooters. She said Lanza and the others share many of the same traits — they were loners who had trouble in school; they rarely had criminal records but often had family issues.

"I'd be looking for answers as to how his behavior evolved and what fueled the anger,'' O'Toole said.

The experts said the person who could have answered some of those questions was his first victim: Nancy Lanza.

"Were there warning signs along the way that she missed?" O'Toole asked. "Frankly, what was Nancy not seeing or didn't want to see? You may see your child do something and explain it or rationalize it."

Both experts said the amount of ammunition and various weapons police found in the Yogananda Street home that the Lanzas shared was stunning.

"The mother and son obviously shared an affinity for guns, and one could certainly question her judgment in giving him access to that,'' Samenow said. "I think looking at the mother is a key element here."

The warrants, edited in some parts, say that investigators found two rifles, a BB gun and a starter pistol in the house, including the .22-caliber rifle that he used to kill his mother before going on his killing spree. Lanza had taken four additional guns with him to the school.

Police also found thousands of dollars worth of computer equipment and video gaming consoles, and a receipt to a gun range in Weatherford, Okla.

A gun safe, where some weapons were stored, was in Adam Lanza's bedroom, a warrant indicates. Investigators found full boxes of shotgun shells with buckshot, hundreds of rounds for the .22-caliber rifle, numerous boxes of ammunition for handguns as well as the instruction manual for the Bushmaster AR-15 used in the shooting.

They also found 12 knives, three samurai swords, a bayonet, eye protection, ear muffs for a gun range, Simmons binoculars and paper targets.

Nancy Lanza, who grew up on a New Hampshire farm girl and was the sister of a police officer, had been around firearms all her life. She bought the Bushmaster semi-automatic rifle and the other weapons used in the Sandy Hook rampage between 2010 and 2012.

Nancy Lanza took Adam Lanza to shooting ranges, and characterized those experiences to close friends as "a way to bond" with her son, who was diagnosed with Asperger's syndrome in middle school.